The production of synthesis gas from the solid and liquid carbonaceous fuels, especially coal, coke, and liquid hydrocarbon feeds, has been utilized for a considerable period of time and has recently undergone significant improvements due to the increased energy demand and the need for clean utilization of otherwise low value carbonaceous material. Synthesis gas may be produced by heating carbonaceous fuels with reactive gases, such as air or oxygen, often in the presence of steam and or water in a gasification reactor to obtain the synthesis gas which is withdrawn from the gasification reactor.
The gasification and subsequent combustion of certain carbonaceous materials provides an environmentally friendly method of generating fuel for power and needed chemicals from these otherwise environmentally unfriendly feedstocks. Coal, petroleum based feedstocks including petroleum coke and other carbonaceous materials, waste hydrocarbons, residual oils and byproducts from heavy crude oil are commonly used for gasification reactions.
Synthesis gas mixtures comprise carbon monoxide and hydrogen. Hydrogen is a commercially important reactant for hydrogenation reactions. The synthesis gas can also be used to generate power from otherwise environmentally unacceptable fuel sources, and as a source of feed gas for the synthesis of hydrocarbons, oxygen-containing organic compounds or ammonia.
Other materials often found in the synthesis gas include hydrogen sulfide, carbon dioxide, ammonia, hydrocarbons, cyanides, and particulates in the form of carbon and trace metals. The extent of the contaminants in the feed is determined by the type of feed and the particular gasification process utilized as well as the operating conditions. In any event, the removal of these contaminants is critical to make gasification a viable process.
As the product gas is discharged from the gasifier, it is usually subjected to a cooling and cleaning operation involving a scrubbing technique wherein the gas is introduced into a scrubber and contacted with a water spray which cools the gas and removes particulates and ionic constituents from the synthesis gas. The initially cooled gas may then be treated to desulfurize the gas prior to utilization of the synthesis gas.
When the product desired is hydrogen, the synthesis gas from the gasifier is shifted using catalyst to form hydrogen as shown below. EQU H.sub.2 O+CO=&gt;H.sub.2 +CO.sub.2
The shift process, also called a water gas shift process or steam reforming, converts water and carbon monoxide to hydrogen and carbon dioxide. The shift process is described in, for example, U.S. Pat. No. 5,472,986, the disclosure of which is incorporated herein by reference.
The hydrogen gas is often used in subsequent processes, particularly hydrotreating. For many applications, especially for hydrotreating hydrocarbons, the hydrogen is required at higher purity and at pressures of between about 1000 psi (6895 kPa) and about 3000 psi (20,684 kPa). The shifted synthesis gas must therefore be purified to meet product specifications.
The synthesis gas is processed to provide a hydrogen rich gas stream and a carbon monoxide/carbon dioxide rich gas stream. Other impurities in the gas generally follow the carbon monoxide/carbon dioxide rich gas stream. One method of purifying gas is via the pressure swing absorption process. This method is expensive and requires significant capital outlay.
A membrane system can also be used to affect the separation. A membrane allows small molecules like hydrogen to pass through (permeate) while the larger molecules (CO.sub.2, CO) do not permeate. Membranes are a cost effective alternative to a pressure swing absorption unit. The membranes reduce the pressure of the product hydrogen so it has to be compressed prior to use. For example, the product hydrogen pressure when purified using a membrane is substantially lower than is required by hydrotreaters.